This invention relates to the coating of printing surfaces with a varnish layer to provide a smooth protective surface. Examples of printing surfaces are paper, metal foil and the surface may be a part of a material, e.g. paper sheet, paperboard, corrugated board or laminated structure.
When the printed surface of a body is printed with a pattern of printing ink or inks, some of which may have hydrophobic surfaces, it is desirable to form a protective layer over the inked surface or the whole treated surface. This varnish layer will protect either the inked surface or the whole surface from physical damage during subsequent handling.
An example of a printing surface is an untreated paper surface which is hydrophilic and liquid compositions based on water will spread readily over the surface. While some printing inks accept water based compositions, other inks form a hydrophobic surface which rejects water based compositions. Thus, when using a water based composition, areas covered by hydrophobic printed inks will reject the composition and a discontinuous varnish layer will be formed. When varnishing paper surfaces in manufacturing facilities the use of water based compositions is preferred because solvent based compositions require fire precautions to be taken. Varnishes coated using lithographic processes are usually based on drying oils. This invention is of particular application to paper surfaces. A paper surface is formed by a mass of discrete cellulosic fibres bonded together by deposition from a fluid medium, for example air or water. Mineral or synthetic fibres may also be present.